The present invention relates to the field of fiber optic communications. More particularly, the present invention relates to the field of filtering of amplified signals used in fiber optic communications systems.
Fiber optic communication systems use optical fibers to carry a modulated lightwave signal between a transmitter and a receiver. A cross-section of a typical optical fiber is illustrated in FIG. 1. The optical fiber 2 includes a core 4 and a cladding 6. Optionally, the optical fiber 2 includes a jacket 8. In a typical optical fiber, the core 4 has an index of refraction greater than the cladding 6, thereby forming an optical waveguide. By maintaining the core diameter within an allowed range, light traveling within the core 4 is limited to a single mode. If included, the jacket 8 protects the outer surface of the cladding 6 and absorbs stray light traveling within the cladding 6. A typical single mode optical fiber intended for use in communication systems operating at a 1300 mn wavelength band or a 1550 nm wavelength band has a core diameter of about 8 xcexcm and a cladding outside diameter of 125 xcexcm. If the jacket 8 is included, the jacket 8 typically has an outside diameter of 250 xcexcm.
In Wavelength Division Multiplexing (WDM) systems, multiple signals are carried by various wavelengths of light through a single optical fiber. A typical WDM system is shown in FIG. 2. The WDM system 10 includes a transmission system 11, which includes a series of transmitters 12, 14, and 16, each coupled to a multiplexer 18. The multiplexer 18 provides an output, which is coupled to an optical fiber 20. Over long distances amplifiers 22 are included along the optical fiber 20. The optical fiber 20 is then also coupled to a receiving system 23, which includes a demultiplexer 24 and a series of receivers 26, 28, and 30. The optical fiber 20 is coupled to an input of the demultiplexer 24 of the receiving system 23. Outputs of the demultiplexer 24 are coupled to the series of receivers 26, 28, and 30.
In the WDM system 10, a first transmitter 12 transmits a light signal at a first wavelength (xcex1), a second transmitter 14 transmits a light signal at a second wavelength (xcex2) and so forth until an nth transmitter 16 transmits a light signal at an nth wavelength (xcexn). The shortest wavelength signal and the longest wavelength signal form a wavelength band. The signals are combined by the multiplexer 18, which then transmits the light signals along the optical fiber 20. Over distance the power of the light signals decrease due to attenuation. The light signals are typically amplified about every 50-100 km. For the 1550 nm wavelength band, this amplification is generally performed by an Erbium Doped Fiber Amplifier (EDFA) 22. When the light signals reach their destination they are separated by the demultiplexer 24. The light signals are then received by the receivers 26, 28, and 30. Light signal intensity versus wavelength for a typical wavelength band of WDM light signals is illustrated in FIG. 3.
Flat gain response for EDFAs is crucial to the performance of the WDM system 10, since small variations in gain for various wavelengths will grow exponentially over a series of in-line EDFAs 22. Agrawal in xe2x80x9cFiber Optic Communication Systems,xe2x80x9d (Wiley, 2nd ed., 1997, pp 414-415) teaches that numerous methods can be used to flatten the gain response of these amplifiers. One method of flattening this gain response is to use channel filters to equalize the gain for various wavelengths. Another method is to adjust the input powers of different wavelengths so that amplification results in uniform intensity for various wavelengths. A third method is to use inhomogeneous broadening of the EDFA gain spectrum to equalize wavelength intensity. A fourth method is to use multiple EDFAs tuned to different wavelength ranges and configured with feedback loops. A final method is to use a filter or series of filters to selectively attenuate the gain response of an EDFA.
A typical gain versus wavelength response for an EDFA is shown in FIG. 4A. When utilizing a filter or series of filters to flatten gain response, an optical filter, with an attenuation curve as shown in FIG. 4B, can be used to selectively attenuate the gain response. The resulting attenuated EDFA gain is shown in FIG. 4C. As shown in FIG. 4C, this attenuated EDFA gain is substantially flat over a range of wavelengths including 1530 nm to 1560 nm. Without a substantially flat gain, the quality of the signal received by the receivers 26, 28, and 30 will be poor.
There are many different known methods for selectively attenuating the gain response of an EDFA in order to improve the signal quality of the signals received by the receivers 26, 28, and 30. U.S. Pat. No. 5,260,823 to Payne et al. entitled, xe2x80x9cErbium-Doped Fibre Amplifier with Shaped Spectral Gain,xe2x80x9d teaches that a wavelength-selective resonant coupling between a propagating core mode to a cladding leaky mode can be used for filtering a wavelength band for EDFA gain flattening. A periodic perturbation of the core forms a grating and the selected wavelength is attenuated by the resonant coupling between the core and the cladding. By varying the perturbation length, various selected wavelengths can be attenuated. Payne et al. also teach that multilayered dielectric coatings can be used for making an optical filter for EDFA gain flattening. A multilayered filtering apparatus includes two coupling lenses and a multilayered dielectric filter. The two coupling lenses connect to an optical fiber and sandwich the multilayered dielectric filter. The multilayered dielectric filter is designed to cancel out the larger gain around the peak wavelength and to be transparent elsewhere.
U.S. Pat. No. 5,473,714 to Vengsarkar entitled, xe2x80x9cOptical Fiber System Using Tapered Fiber Devices,xe2x80x9d teaches that tapered fiber devices can be used for filtering in an optical telecommunications system. Vengsarkar teaches that by tapering an optical fiber, light can be attenuated by wavelength cutoff and direct coupling from a core to a cladding. The tapered fiber device is formed from the optical fiber by heating the optical fiber and stretching it. The taper reduces the diameter of the core to a value close to the cutoff wavelength. Light with wavelengths near and above the cutoff wavelength are coupled directly to the cladding.
U.S. Pat. No. 5,583,689 to Cassidy et al. entitled xe2x80x9cFilter With Preselected Attenuation/Wavelength Characteristic,xe2x80x9d teaches that a fiber grating, with spatially separated parts having different attenuation characteristics, can perform filtering for EDFA gain flattening. The fiber grating is preferably a side-tap Bragg fiber grating. By varying the pitch along the fiber grating an appropriate attenuation profile can be provided for flattening the EDFA gain response.
U.S. Pat. No. 5,067,789 to Hall et al. entitled, xe2x80x9cFiber Optical Coupling Filter and Amplifier,xe2x80x9d teaches that a light-attenuating light path adjacent to a first core within a cladding can be used to filter wavelengths about a specific wavelength for EDFA gain flattening. The light attenuating light path is preferably one or more lossy cores that are evanescently coupled to the first core. The evanescent coupling between the first core and the light attenuating light path is greatest where the effective index of refraction of the first core equals the effective index of refraction of the light attenuating light path. By choosing a single mode or a higher multimode optical waveguide structure for the light attenuating light path, the effective index of refraction for the light attenuating light path can be varied. Hall et al. teach that the index of refraction for the material for the light attenuating light path should be greater than the index of refraction for the material for the first core. Hall et al. further teach that as an alternative embodiment the lossy core could be a lossy annular region located concentrically about the first core and within the cladding. A necessary feature of this filter is that the lossy core or the lossy annular region has specific light absorption characteristics. Since the lossy core or the lossy annular region is contained completely within the cladding, the specific light absorption characteristics dissipates light energy that has been filtered from the first core to the lossy core or the lossy annular region. The absorption characteristics of the lossy core or the lossy annular region determine an amount of attenuation of the filtered wavelengths.
Each of these known methods for filtering an amplified signal from an EDFA can be inefficient, unreliable, and expensive. There is currently a lack of efficient filters for gain flattening in fiber optic systems, which are easy to manufacture and use within a WDM system.
An all fiber optical filter is formed by stretching an optical fiber. The all fiber filter includes a core, an inner cladding, and an outer cladding. A core index of refraction is greater than an outer cladding index of refraction. The outer cladding index of refraction is greater than an inner cladding index of refraction. The all fiber optical filter attenuates a portion of an optical signal by transferring optical energy from the core to the outer cladding by evanescent coupling. The all fiber optical filter has a compact structure, which prevents bending and provides stable temperature performance.
The all fiber optical filter is preferably used in Wavelength Division Multiplexing (WDM) systems for gain flattening of gain responses from Erbium Doped Fiber Amplifiers (EDFAs). Alternatively, the all fiber optical filter is used in other applications where optical filtering or attenuation is needed.
The all fiber optical filter is manufactured by holding a length of an appropriate optical fiber between two clamps, heating the optical fiber, and stretching the optical fiber until a predetermined characteristic of the optical fiber is achieved.